1.) Causes For Valve Faliure?
Valve is one of the most critical components of Valve-train mechanism of an
Engine. Since it is subjected to extreme mechanical & thermal stresses,
there has been considerable advances in Valve designs, material as well as
production techniques.

For a typical Petrol Engine of @ 5400 RPM, temperatures at Exhaust ports
go as high as 700 ° C; while Valve opens & closes 45 times in a second.
Because of such extreme working requirements, Valves have to be of absolutely
good Quality with regards to workmanship, material, etc. Few of the major
causes of Valve failure are as mentioned below:

Deposits resulting from residue of combustion viz. Carbon & unburnt
fuel along with lubricating oil opens out the end of the Valve Guide. As
the combustion progresses over a period of time, because of higher temperatures,
more & more deposit is forced into the conical opening leading to restricted
movement of Valve Stem in the guide which leads to incorrect closure of
valve head. This leads to exposure of Valve head in the chamber for a longer
duration at excessive temperature, which may lead to rapid failure.

Sometimes it so happens that these residual deposits, because of higher
temperatures get soft & sticky; they break-off & get stuck between
Valve Seat & Cylinder head. Because of a constant hammering action,
a depression is created which leads to leaks. This in turn may lead to damage
of valve seat portion known as “burning“ which causes excessive
depressions & subsequent partial or complete destruction of Head.

A too lean mixture of Air with Fuel in carburetion, for improving the
fuel efficiency, may increase the combustion time leading to overheating
at exhaust valve port.

For no reason the engine should be overheated as it leads to excessive
temperatures at exhaust valve port & subsequent rapid failure of Valves.

It is absolutely vital to use a proper recommended grade of engine oil.
Oil provides proper lubrication to reduce friction as well as carries away
some heat. Since oil comes in contact with almost all the vital components
of the engine, a wrong grade or dirty oil with too many suspended particles,
would lead to rapid failure of not only valves but also bearings, crankshaft
etc. Hence the emphasis on good clean oil for prolonged engine life is very
justified.

The life of valve is also dependent on the quality & condition of
its adjoining components like Valve Guide, Valve Seat Insert, spring, Collet,
etc.